Measurement-While-Drilling (MWD) electromagnetic propagation based borehole formation evaluation tools for use in oil and gas well drilling applications are known. Typically, electromagnetic based evaluation tools include a housing, which forms part of the drillstring, a transmitting antenna mounted in the housing and a pair of receiving antennas mounted in the housing. Typically, each of the antennas takes the form of a wire loop secured within a circumferential groove surrounding the housing. Each antenna is embedded in a layer of an elastomeric material to protect the antenna from shock, vibration and moisture during the drilling process. An electromagnetic wave is propagated from the transmitting antenna into the formation surrounding the borehole and is detected as it passes the receiving antennas. The signals received are used to calculate electrical properties, e.g., permitivity, resistivity, of the formation.
The elastomeric layer surrounding the antenna is vulnerable to wear, abrasion and adhesive failure between the layer and the tool housing, any one of which may lead to failure of the layer and its associated antenna.
Repair of a failed antenna is particularly burdensome, in that the formation evaluation tool must be removed from the borehole and shipped from the drilling site to a repair facility. This results in considerable downtime and expenses and necessitates maintaining an inventory of replacement tools at the drill site.